The present invention relates to an automatic volume control apparatus, called, for example, an ASL (automatic sound levelizer), for use in a mobile audio apparatus.
A mobile audio apparatus often suffers from ambient noise problems from, for example, engine noise and road noise generated between the tires and road surface. The reproduced sound from the apparatus suffers from a masking effect due to such noise. Accordingly, it is necessary to perform volume control corresponding to the level of noise.
FIG. 1 is a diagram illustrating a conventional mobile automatic volume control apparatus. In FIG. 1, reference numeral 1 represents a signal line receiving an audio signal, 2 designates a sound volume control section receiving the audio signal as an input thereto, 3 is a power amplifier for power-amplifying the output of the volume control section 2, and 4 represents a speaker for converting the output from the power amplifier 3 into an audio signal. Reference numeral 5 indicates a microphone for detecting background noise, 6 is a noise detection section receiving the output of the microphone 5 as an input thereto, 7 represents a diode with its cathode and anode connected to the output of the noise detection section 6 and a control signal input of the volume control section 2, respectively, 8 designates a resistor connected to the diode 7 in parallel thereto, and 9 is a capacitor 9 connected between the control signal input of the volume control section 2 and a reference potential point (ground).
In the above-described arrangement, the audio signal received on the signal line 1 is volume-controlled in the volume control section 2, power-amplified by the power amplifier 3, and then reproduced as sound rom the speaker 4. If the amplification factor of the volume control section 2 were controlled directly according to a noise voltage obtained from the microphone 5, the resulting extreme variations in volume would be annoying. To avoid this problem, a time constant is provided by a low-pass filter constituted by the resistor 8 and the capacitor 9 so that a more gently varying voltage produced across the capacitor 9 is employed for the control voltage for the volume control section 2. However, the time constant provided by the filter constituted by the resistor 8 and the capacitor 9 has the same characteristic for both noise level increases and decreases.
Since the perceived abnormality in sound volume can be reduced if the time constant for noise level decreases is made shorter than that in the case of noise level increase, a diode 7 is connected in parallel with the resistor 8 as shown in FIG. 1. (In this circuit, it is assumed that the output voltage of the noise detection section 6 rises as the noise level increases.)
The diode 7 is reversely biased when the output of the noise detection section 6 is rising so that no current flows through diode 7. The time constant when the noise level is rising therefore depends on the component values of the resistor 8 and capacitor 9. When the noise level is falling, the diode 7 is forwardly biased so that the equivalent resistance value of the resistor 8 is reduced. Therefore, the time constant when the noise level is falling is shorter than when the noise level is rising.
In the prior art arrangement shown in FIG. 1, the volume rises gently when the noise level is rising but quickly when the noise level is falling. The time constant, however, depends on resistance and capacitance values, while the voltage applied to the volume control section 2 as a control signal changes along an exponential curve. Although there is no large sense of abnormality within a region of small noise level changes, when a large noise level change occurs, for example, in the case of a sudden stop, there is a remarkable difference in the rate of change in volume between the initiation and in the termination of the change in volume so that significant sense of abnormality occurs. That is, for a noise level decrease, the terminal voltage of the capacitor 9 is discharged through the low impedance of the diode in its on-state. When the noise level drops, however, the current through the diode 7 decreases as the terminal voltage of the capacitor 9 approaches the noise voltage at that point of time, causing the impedance across the diode 7 to decrease. If this occurs at the same time that the control signal is changing along the final portions of the exponential curve, the perceived abnormality in the sound level change is remarkable.
FIG. 2 is a diagram illustrating another example of a conventional mobile automatic volume control apparatus. In FIG. 2, reference numeral 101 represents a microphone 101 for detecting the ambient noise level, and 102 is a noise detection section 102 constituted by an amplifier for amplifying the output of the microphone 101, a low-pass filter for extracting a low-band component from the output of the amplifier, a detector for AM-detecting the output of the filter, and related components.
The cutoff frequency of the low-pass filter in the noise detection section 102 is set at about 10 Hz taking into consideration the spectrum of the audio signal and the vibration frequency of a typical vehicle engine. (For example, the vibration frequency is about 27 Hz in the case of a four-cycle, four-cylinder engine at an idling speed of 800 rpm.)
Reference numeral 103 designates an audio signal input terminal to which a reproduction output from a tuner, tape deck or the like is applied. The reproduction output is applied to an audio signal detection section 104. The audio signal detection section 104, constituted by an envelope detector, detects the input level of the audio signal applied to the input terminal 103.
The outputs of the noise detection section 102 and the audio signal detection section 104 are respectively applied to logarithmic amplifiers 105 and 106 having the same characteristic, and then applied to a subtractor 107 after being compressed logarithmically by the logarithmic amplifiers 105 and 106. Thus, the subtractor 107 receives noise detection information from the logarithmic amplifier 105 as a positive input and audio signal detection information from the logarithmic amplifier 106 as a negative input. The differential output of the subtractor 107 is applied to a volume controller 108 as a control signal. The volume controller 108 is constituted by a variable gain amplifier. The signal applied to the audio signal input terminal 103 is applied to a control input of the volume controller 108, whereby the level of the audio signal is controlled according to the output of the subtractor 107. The audio signal having a level controlled by the volume controller 108 is supplied to an output terminal 109, amplified by a main amplifier (not shown), and reproduced by a speaker. The volume controller 108 responds to a linear increase/decrease of the control signal applied to its control input so as to increase/decrease the gain logarithmically in accordance with the ambient noise level.
Being arranged as described above and operating on the assumption that all ambient noise is confined in frequency below 10 Hz, this conventional automatic volume control apparatus has had a problem in that it is impossible to deal with variations in audible frequency components of noise occurring upon changes in vehicle speed. That is, in actual use, volume control is carried out primarily in response to changes in audio level rather than noise.